[Blazor] Include mouse position in page coordinates in UIDragEventArgs

[SamVogelskamp]

Expand the current UIDragEventArgs by pageX and pageY variables.

[mkArtakMSFT]

@danroth27, FYI regarding the milestone and priority assignment.

[rynowak]

This would include providing extra information on the event args in addition to what the DOM provides. While this is probably the most straightforward way to proceed, we've considered it in the past and didn't do it right away.

Doing something like this would involved a more thorough look at the events in general and trying to understand what else we would want to polyfill because there are a few notable cases (scrolling is similar).